What is the max concurrent tasks a central host will support?

We have thousands of tasks defined across several projects on a single central host. 

Each of the projects have dedicated Task Machines to them - limited at the project level, but also via project and task type filtering on the task machine.

I observe that when one project has kicked off a large amount of tasks, other project tasks will sit in a new status - even though their respective task machine is sitting there twiddling thumbs. 

It feels like that when the task machine is querying for tasks, it is performing something similar to a "select top 1000" .. and unless there are tasks in that bunch, the task machine will do nothing.

Thanks